I've seen this done with a 3d lattice of marshmallows connected with toothpicks. It was done without the carousel, so the finished product gives you a nice 3d model of where the microwaves are.
Hard to really see what the microwaves are doing, though.
Ideally I suppose you'd want to film it with the array in various z-positions from the rear to the front, then composite all the versions together, perhaps with each version given a color overlay 'in post' to keep them distinct. Probably want to disable the microwave's interior light, and have the microwaved bulbs be the only source of light.
It's simple to avoid if you're not intending to drink the glass of water, simply drop a toothpick in - break it if you want to be extra sure. The toothpick has a rough surface, if you break it then you're making an exceptionally rough nucleation point between the jarred fibers.
IIRC it's not so much a clean glass as a polished glass. If it's a regular cheap ass glass then it has enough imperfections that it's unlikely to ever happen with tap water. However polished glass (where the inside is actually sanded/polished smooth to remove imperfections) is the dangerous one, especially with distilled water (why anyone would drink distilled I don't know) or I suppose even filtered water would increase the risk exponentially compared to tap that can contain sediments from pipe scum.
